Process for the selective absorption of chlorine and/or bromine from CO.sub. -containing off-gases with the aid of steam

ABSTRACT

The invention provides a process for the selective absorption of chlorine and/or bromine from CO 2  -containing off-gases. The off-gas must contain at least 1 mol of SO 2  per mol of Cl 2  and/or per mol of Br 2  ; if necessary sufficient SO 2  is added to the off-gas to give the above-mentioned relative amount. Chlorine and/or bromine is absorbed by mixing the off-gas with steam and then cooling the mixture until the steam condenses. An example of a suitable apparatus is a vertical tube having an internal cooling coil.

This application is a continuation of our co-pending application Ser. No. 07/546,102, filed Jun. 29, 1990, now abandoned.

The combustion of chlorinated hydrocarbons gives, besides hydrogen chloride and CO₂, according to the DEACON equilibrium, also elemental chlorine. This chlorine is not allowed to be discharged into the environment and must consequently be removed to the greatest possible extent from the off-gas. In the Federal Republic of Germany, the chlorine concentration must be below 5 mg/³.

The absorption of chlorine from gas streams has hitherto generally been carried out using sodium hydroxide solution with the formation of sodium hypochlorite:

    Cl.sub.2 +2NaOH→NaCl+NaOCl+H.sub.2 O

In the case of gas streams which besides chlorine contain only inert gas, an industrially usable product (NaOCl solution) is obtained from the absorbent. If the off-gas which is to be absorbed contains other alkali-soluble gases besides Cl₂, the identical procedure gives a solution of several salts which is virtually unusable. For example, CO₂ is also absorbed by sodium hydroxide solution with the formation of NaHCO₃ :

    NaOH+CO.sub.2 →NaHCO.sub.3

According to the prior art, NaOCl is removed from these mixtures of salts by adding Na₂ SO₃.

Minimization of the troublesome simultaneous absorption of CO₂ has already been given much attention, so that techniques are already known for specific applications.

A process described in DE-A-2,849,498 employs two absorption cycles operated using NaOH for absorbing Cl₂ to give NaCl and NaOCl. A portion of the resulting NaOCl is continually discharged and decomposed using NaHSO₃. However, the off-gas purified according to this process still contains about 10 ppm, i.e. about 30 mg/m³, of chlorine.

The multi-stage countercurrent absorption of chlorine using sodium hydroxide solution, which has been described in U.S. Pat. No. 3,984,523, is suitable only for gas streams which are firstly highly loaded with chlorine and secondly contain CO₂ in a similar concentration to that of Cl₂. However, owing to the high absorption of CO₂ in alkali, this process is unsuitable for use with a 100-1,000-fold excess of CO₂ relative to Cl₂ in the off-gas. At high chlorine loadings in the off-gas, the CO₂ which has been absorbed in the last stage is driven out again by chlorine in the first stage, a procedure which is not possible at low chlorine loadings.

Furthermore, it is known from the Derwent Report 3452V/42, referring to Japanese Application 72/84607 (Publication No. 74/40,277) to absorb Cl₂ from CO₂ -containing off-gases in a 3-stage process by first washing out the bulk of the Cl₂ with sea water in an absorption tower, then reacting the remaining Cl₂ with SO₂, and then washing again with water. In this process, the second washing is carried out using a jet scrubber.

Surprisingly, it has now been found that Cl₂ and/or Br₂ can be absorbed from a CO₂ -containing off-gas with the aid of condensing steam if the off-gas contains at least mol of SO₂ per mol of Cl₂ and/or per mol of Br₂. If the off-gas does not originally contain such an amount of SO₂, sufficient SO₂ is added to the off-gas to comply with this

The present invention accordingly provides a process for the absorption of Cl₂ and/or Br₂ from CO₂ -containing off-gas which contains at least 1 mol of SO₂ per mol of Cl₂ and/or per mol of Br₂, which comprises carrying out the absorption with the aid of condensing steam.

The invention also provides a process for the absorption of Cl₂ and/or Br₂ from CO₂ -containing off-gas which contains less than 1 mol of SO₂ per mol of Cl₂ and/or per mol of Br₂, wherein sufficient SO₂ is added to the off-gas to give at least 1 mol of SO₂ per mol of Cl₂ and/or per mol of Br₂ and then the absorption is carried out with the aid of condensing steam.

1 mol of SO₂ per mol of Cl₂ and/or per mol of Br₂ here denotes 1 mol of SO₂ per mol of halogen which may be Cl₂ and/or Br₂.

Cl₂ and/or Br₂ are absorbed by mixing the off-gas, optionally after adding SO₂, with steam and then cooling the mixture until the steam condenses. The degree of absorption can be adjusted using the amount of steam and the intensity of cooling.

An example of a suitable apparatus for the absorption is a vertical tube having an internal cooling coil. The off-gas, optionally containing added SO₂, and (preferably slightly superheated) steam are introduced at the head of the tube. Cooling water is passed through the cooling coil, a long column of mist forming in which the absorption takes place. The condensed water droplets form drops which are collected at the foot of the tube and drawn off. Then further water is condensed out from the off-gas steam mixture by cooling the mixture to about 20° C. This can be carried out with the aid of a second condenser or of a conventional absorption column which is operated using the collected condensate from the upstream-connected condenser. After this, the off-gas is substantially free of Cl₂ or Br₂ while the CO₂ is not absorbed.

The process according to the invention is suitable in particular for CO₂ -rich off-gases, even for those whose CO₂ -content is 10 to 10s times as high as the Cl₂ content or Br₂ content.

The invention is explained by the example which follows.

EXAMPLE 1

An off-gas containing 24.8% by weight of CO₂, 75% by weight of N₂, 0.1% by weight of Cl₂ (=1,000 ppm) and 0.1% by weight of SO₂ (=1,000 ppm) was purified. The off-gas flowed through two condensers (tubes with internal cooling coils) connected in succession, the gas passing from top to bottom in each case.

The cooling coil of the first condenser carried thermostatically controlled water at a temperature of 70° C. The cooling coil of the second condenser carried thermostatically controlled water at a temperature of 15° C. The volume flow passed through the apparatus was 1.8 m³ /h. To this off-gas were added about 1,100 g/h of steam at a temperature of 100° C. at the head of the first condenser. The concentration of Cl₂ measured at the base of the second condenser was about 1 ppm of Cl₂ in the purified off-gas, the SO₂ concentration being less than 1 ppm.

The degree of absorption of Cl₂ was about 99.9%, and the degree of absorption of SO₂ was even higher. 

We claim:
 1. A process for the absorption of Cl₂ or Br₂, or mixtures thereof, from CO₂ -containing an off-gas which contains at least 1 mol of SO₂ per mol of Cl₂ or per mol of Br₂, or mixtures thereof, which comprises mixing the off-gas with steam and cooling the mixture until the steam condenses to effect a contact of water droplets with the gas, and obtaining as a result a purified off-gas still containing CO₂ but containing no more than about 5 mg of Cl₂ or Br₂, or mixtures thereof, per m³ purified off-gas.
 2. The process as claimed in claim 1, wherein the steam is superheated.
 3. A process for the absorption of Cl₂ of Br₂, or mixtures thereof, from CO₂ -containing off-gas which contains less than 1 mol of SO₂ per mol of Cl₂ or Br₂, or mixtures thereof, wherein sufficient SO₂ is added to the off-gas to give at least 1 mol of SO₂ per mol of Cl₂ or per mol of Br₂, or mixtures thereof, mixing the SO₂ -containing off-gas with steam and cooling the mixture until the steam condenses to effect a contact of water droplets with the gas, and obtaining as a result a purified off-gas still containing CO₂ but containing no more than about 5 mg of Cl₂ or Br₂, or mixtures thereof, per m³ of purified off-gas.
 4. The process as claimed in claim 3, wherein the steam is superheated. 